Tale of the Tuna

Understanding the movement of Fukushima-derived radioactivity through marine ecosystems may come down to getting a better handle on the tiniest of creatures—the microscopic plankton that take up so much volume in the sea. But one species that has become emblematic of the disaster is a shimmering giant: the Pacific bluefin tuna.

Increasingly overfished, Pacific bluefins are among the most prized table fish in the world. A single 500-pound specimen recently fetched $1.76 million in a Tokyo auction. Beyond their allure as high-end sushi material, however, they are amazing migratory animals. Spawned in the waters off Japan and the Philippines, these fish as juveniles swim the entire 6,000-mile breadth of the Pacific—a four-month journey—to fatten up in food-rich waters off California. Years later, larger and sexually mature adults undertake a return crossing to spawn.

As respective experts on radioisotope uptake in marine life and tuna migration patterns, Nicholas Fisher of Stony Brook University and Daniel Madigan of Stanford’s Hopkins Marine Station knew that young bluefins caught off California during the summer of 2011 likely would have spent their early days in contaminated waters off Fukushima. Would these fish act as “biological vectors” transporting radioisotopes between distant shores?

To find out, Fisher and Madigan obtained tissue samples from tuna caught by sport fishermen off San Diego in August 2011, and analyzed them in Fisher’s lab. “Every single bluefin we tested—15 out of 15—had both cesium-134 and cesium-137”—telltale evidence of contamination from the damaged Dai-ichi nuclear power plant, Fisher said. “We were quite surprised to see that.”

The radiation levels they measured were “very, very low,” Fisher stressed. In bluefin tuna caught off San Diego, the total radioactive cesium levels were 10 becquerels, only three percent above radiation levels from naturally occurring potassium-40, and far below safe-consumption levels set by the United States and Japanese governments.

Estimating that the migrating tuna would have lost two percent of any absorbed cesium per day as they crossed—but also would have picked up traces of Cold War-era cesium-137 during their journey—Fisher and his colleagues back-calculated that when the fish left Japanese waters, the concentrations in them were likely to be 15 times higher, about 150 becquerels per kilogram.

Fisher and Madigan ruled out the possibility that the cesium they measured had been carried on ocean currents or through the atmosphere by also sampling yellowfin tuna, which reside off California but do not migrate across the Pacific Ocean. They found no cesium-134 in these fish, and only background levels of cesium-137.

Fisher and Madigan published their results in late May of 2012, and the response was titanic. “Seven hundred U.S. newspapers, and 400 elsewhere, carried this story,” Fisher recalled, “often on the front page.” He submitted to countless interviews and made several television appearances to try to explain his findings.

“People were genuinely terrified of radioactivity,” he said, “and yet few people could even define it.” To address the anxiety, Fisher and French colleagues calculated dosages, comparing the radiation a person would ingest from eating these bluefin tuna (0.000008 millsieverts) to that received from eating a banana, with its natural potassium (0.0001 mSv), getting a dental X-ray (0.005 mSv), or taking a transcontinental flight (0.04 mSv). He would be more concerned about the health impacts of mercury in these fish, he said, than about radiation.

The scientists continue to analyze radioactivity in bluefins, and Madigan, Fisher, and Zofia Baumann, also in Fisher’s group, recently reported that bluefins caught off San Diego in 2012 had less than half the radioactive cesium of the 2011 tuna, indicating that radioactive cesium concentrations in tissues were indeed declining.

But to Fisher, the real importance so far of these findings is that the presence of Fukushima radioisotopes could be used as unequivocal tracers of migratory patterns of bluefin tuna and possibly other large migratory animals such as sharks, seabirds, and loggerhead turtles. And understanding the timing and routes of migration patterns can help manage fisheries and devise more effective conservation strategies for threatened species.

From the Series

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